This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to a Japanese Patent Application 2002-150237, filed on May 24, 2002, the entire content of which is incorporated herein by reference.
This invention generally relates to an antiskid braking control system for a vehicle for controlling braking force applied to a vehicle wheel such that the vehicle wheel is prevented from being locked and skidding upon vehicle braking operation. More particularly, this invention pertains to an antiskid braking control system for a vehicle for performing antiskid braking control with reference to an estimated grip factor indicative of a grip level of tire on a road surface in a lateral direction of a vehicle wheel.
An antiskid braking control system is generally employed for preventing a vehicle wheel from being locked by increasing, maintaining, or decreasing wheel brake cylinder pressure. The wheel brake cylinder pressure control is performed based upon a slip ratio of the vehicle wheel (a vehicle wheel tire), a vehicle wheel acceleration (including a vehicle wheel deceleration), and the like.
Disclosed is an antiskid braking control system for an automobile in a Japanese Patent Laid-Open Publication No. 8-85437. This disclosed antiskid braking control system is employed for adjusting a start-up timing of pressure decrease when a vehicle is judged to have turned with an acute turning angle based upon a lateral acceleration, thereby enabling to perform smooth and stable braking operation upon the vehicle sharp turn. More particularly, the braking pressure is decreased or maintained at a normal start-up timing of the pressure decrease when the lateral acceleration of the vehicle does not exceed a predetermined level for judging the vehicle sharp turn. On the other hand, the braking force is decreased or maintained at an earlier timing than the normal start-up timing of the pressure decrease when the lateral acceleration of the vehicle exceeds the predetermined level.
Disclosed is a road frictional coefficient detecting device in a Japanese Patent Publication No. 3166472. According to a known work for detecting a road frictional coefficient disclosed in a Japanese Patent Laid-Open Publication No. 60-148769 which is cited in this Patent Publication, cornering force of a vehicle wheel is saturated when a lateral slip angle of the vehicle is increased to a certain angle amount during the vehicle turning and a saturation value of the cornering force corresponds to a road surface xcexc. Therefore, the road surface xcexc can be obtained based upon the saturation value of the cornering force. However, the above-described Patent Publication indicates therein a problem of the known work that the road surface xcexc may be able to be detected only when a vehicle wheel grip condition is approximated to a limit value. In other words, it may be difficult for the known work to detect the road surface xcexc prior to the approximation of the vehicle wheel grip condition to the limit value. Further, according to the above-described Patent Publication, a relationship between a vehicle return moment and the cornering force as a reference for detecting the road surface xcexc enables to more effectively detect the road surface xcexc even in a zone with a relatively small cornering force rather than referring to the relationship between the vehicle lateral slip angle and the cornering force.
According to the above-described Japanese Patent Publication No. 3166472, when the detecting precision of the frictional coefficient is assumed to be relatively low, a final road frictional coefficient of a previous time is used as a final road frictional coefficient of every time. Further, the final road frictional coefficient of every time is assumed to be mostly set at the final road frictional coefficient of the last time which was detected during the road frictional coefficient being detected with a relatively high precision prior to the deteriorating of the detecting precision thereof. In the aforementioned assumptions, it was an objective of the invention disclosed in the Japanese Patent Publication that the road frictional coefficient be properly detected even when the detecting precision of the road frictional coefficient is relatively low in order to achieve the objective, the road surface xcexc during the detecting precision being relatively low should be determined in relation to designing of a vehicle control system, and the road surface xcexc of every time detected during the detecting precision being relatively low should be determined to be approximated not to a previous value but to a predetermined value.
In the above-described Japanese Patent Publication, disclosed is the road frictional coefficient detecting device which determines the final road frictional coefficient (a final value of this time) depending upon the detecting precision for detecting the road frictional coefficient. The detecting precision for detecting a provisional road frictional coefficient (a provisional value of this time) is estimated based upon the relationship between the return moment and the cornering force which were detected this time. When the estimated detecting precision is improved, the final value of this time is approximated to the provisional value of this time based upon at least the return moment and a predetermined road frictional coefficient. On the other hand, when the estimated detecting precision is deteriorated, the final value of this time is approximated to the predetermined road frictional coefficient.
According to a generally known antiskid braking control system, control parameters including an antiskid braking control starting threshold value are set corresponding to the road frictional coefficient (i.e. the road surface xcexc). In this case, the maximum value of the road surfaced xcexc is required. Therefore, the road surface xcexc is used as the maximum road frictional coefficient unless there is particular description about the road frictional coefficient. However, according to the above-described Japanese Patent Laid-Open Publication No. 1996-85437, the lateral acceleration is referred to to judge whether or not the vehicle has turned with an acute turning angle. For example, even when the vehicle is turning on a low-friction (low xcexc) road with friction force somewhere around a limit thereof, the lateral acceleration of the vehicle is relatively large. In this case, the braking pressure is decreased or maintained at the normal start-up timing of the pressure decrease. On the other hand, when the vehicle is turning with a relatively large grip force on a high-friction (high xcexc) road, there is still a margin to the limit of the friction force. However, the braking pressure is decreased or maintained at an earlier timing than the normal start-up timing of the pressure decrease.
When the vehicle is traveling, the vehicle is slipped due to difference between a vehicle traveling speed, i.e. a tire traveling speed and a tire peripheral velocity. A longitudinal force hence is generated in response to a so-called slip ratio. Further, a slip angle is generated due to a difference between a vehicle traveling direction, i.e. a tire traveling direction and a tire oriented direction. A lateral force is hence generated in response to a so-called front wheel slip angle. The vehicle is accelerated, decelerated, or turned in a flat surface being parallel with the road in accordance with the longitudinal force and the lateral force. Therefore, vehicle turn characteristics are not uniform even on a uniform surface. The vehicle turn characteristics vary due to the road conditions, and the relationship between the road surface and the tire.
According to the road frictional coefficient detecting device disclosed in the aforementioned Japanese Patent Publication No. 3166472, the final road frictional coefficient of this time is approximated to the predetermined road frictional coefficient when the detecting precision is deteriorated. Therefore, the maximum road frictional coefficient can be estimated as accurately as possible when the detecting precision is deteriorated. This type of road frictional coefficient detecting device can be applicable for the antiskid braking control system. However, the maximum value of the road frictional coefficient can not be estimated at a proper timing for adjusting the control parameters for the antiskid braking control system.
The maximum value of the road frictional coefficient cannot be obtained at the present moment while the vehicle has been traveling. Therefore, it may be difficult to comprehend ratio of the road frictional coefficient at the present moment relative to the maximum frictional coefficient. For example, when the side force is applied to the tire somewhere around 0.2G, the tire grip factor varies depending upon the condition of the road of which acceptable maximum side force is somewhere around 1G or 0.5G. That is, the grip factor of the vehicle front wheel varies in accordance with the acceptable side force. In the known antiskid braking control system, the maximum value of the road frictional coefficient is estimated such that the control parameter is set for controlling pressure reduce with reference to the maximum value of the road frictional coefficient. However, the braking force can be effectively controlled at an earlier timing prior to the set of the control parameter if the grip factor can be estimated prior to the set of the control parameter.
The present invention therefore seeks to provide properly setting a control parameter at an earlier timing prior to the reach to the limit of the friction force such that an antiskid braking control can be-performed assuring stable braking operation even while the vehicle has turned.
According to an aspect of the present invention, an antiskid braking control system includes a braking force generating means for generating braking force applied to a vehicle wheel, a braking force controlling means for controlling the braking force generated by the braking force generating means, and a vehicle wheel speed detecting means for detecting a vehicle wheel speed. A control parameter for the braking force controlling means is determined based upon at least the vehicle wheel speed detected by the vehicle wheel speed detecting means, and the braking force controlling means is controlled in accordance with the determined control parameter.
The antiskid braking control system further includes a steering factor detecting means for detecting at least one of steering factors including a steering torque and steering effort applied to a steering system extending from a steering wheel to a suspension of a vehicle, an aligning torque estimating means for estimating aligning torque produced on at least a wheel of the vehicle on the basis of the steering factor detected by the steering factor detecting means, a vehicle state variable detecting means for detecting a state variable of the vehicle, a wheel factor estimating means for estimating at least one of wheel factors including a side force and slip angle applied to the wheel on the basis of the state variable detected by the vehicle state variable detecting means, a grip factor estimating means for estimating a grip factor of at least a tire of the wheel, in accordance with a relationship between the alignment torque estimated by the aligning torque estimating means and the wheel factor estimated by the wheel factor estimation means, and a control parameter adjusting means for adjusting the control parameter based upon the grip factor estimated by the grip factor estimating means. The vehicle state variable includes a variable associated with a driven vehicle such as a vehicle speed, a lateral acceleration, a yaw rate, a steering angle, and the like.
The antiskid braking control system further includes a reference aligning torque setting means for setting a reference aligning torque based upon the wheel factor estimated by the wheel factor estimating means and the aligning torque estimated by the aligning torque estimating means. The grip factor estimating means estimates at least the grip factor of the vehicle front wheel based upon a comparison between the reference aligning torque and the aligning torque.
The control parameter adjusting means adjusts a control start threshold value for staring the braking force control by the braking force controlling means based upon the grip factor estimated by the grip factor estimating means. Further, the control parameter adjusting means adjusts a control amount of the braking force by the braking force controlling means based upon the grip factor estimated by the grip factor estimating means. Especially, it is preferable that the braking force applied to vehicle rear wheel is adjusted based upon the grip factor.